Make auto_gen_config_spec.rb more stable
Make auto_gen_config_spec.rb more stable

The previous commit caused failures in this file for some ruby versions in the
CI builds. I have not been able to reproduce locally, but I suspect that since
DisabledConfigFormatter has two singleton variables that keep their values
between test cases, we need to clear both of them.

bbatsov/rubocop,rrosenblum/rubocop,maxjacobson/rubocop,tejasbubane/rubocop,bbatsov/rubocop,maxjacobson/rubocop,rrosenblum/rubocop,tejasbubane/rubocop,maxjacobson/rubocop,rrosenblum/rubocop,tejasbubane/rubocop